Admins can delete lessons

**As an** admin
**I want to** delete a lesson
**so that I can** remove lessons that are no longer used.

## Technical Requirements
- An API endpoint, `/api/lessons/:lessonId`, which accepts a `DELETE` request. It should delete the requested lesson and return a success/error message. (Server)
- On the lessons list, each lesson should have a delete button that sends the delete request to the server. (Client)

## Acceptance Criteria
- [x] I can browse to the list of lessons.
- [x] I can click on a delete button next to each lesson.
- [x] When I click on the delete button next to each lesson, I should see a success or error message.
- [x] When a lesson has been deleted, it should be removed from the list.